Lennox furnace Alert code 271: Low pressure switch open — Watchguard
Alert 271 means the furnace has entered Watchguard mode after five failed attempts to prove draft on the low pressure switch during one demand.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
This is the escalated form of the low pressure switch alert. Rather than reporting a single failure to prove draft, it reports that the furnace tried five times within one heating demand and stopped. Lennox rates it Service Urgent, a step above the non-Watchguard version, which reflects that a repeated failure is a consistent condition rather than a transient one. The checklist is the same familiar set — vent blockage, inducer operation, cold end heater box and drain, cracked hoses, levelness — but the five strike count means a marginal or intermittent explanation is less likely than a real restriction.
Most common causes
- A blocked vent or intake at the outside termination.
- A blocked cold end heater box, condensate drain, or drain strainers.
- Cracked hoses in the pressure switch circuit.
- The unit not level, so condensate does not drain as designed.
- A weak or failing combustion air inducer.
- A pressure switch not closing at its rated pressure.
- Vent piping restricted or incorrectly sized.
How to diagnose it
- Look at the outside vent termination first — highest yield and no tools needed.
- Check the condensate drain for blockage.
- Note that this alert means five consecutive failures, so a one-off explanation such as a gust of wind is unlikely.
- Note the weather, since freezing at the termination produces exactly this.
- Lennox references Service and Application Note H-13-07 for additional detail.
How to fix it
- Clear the vent termination and condensate drain if either is blocked. Both are homeowner-reachable and both are on Lennox's list.
- The pressure switch, hoses, inducer, and cold end heater box are technician work.
- If the unit is not level, that needs correcting properly.
- Never bypass a pressure switch. It is what prevents the furnace firing into a flue that cannot carry the products away.
Notes
The Watchguard alerts are worth distinguishing from their non-Watchguard counterparts. The same underlying fault reported after five strikes means something consistent is wrong, which narrows the plausible causes considerably.
Related
- Alert code 223 — Low pressure switch open
- Alert code 272 — Low pressure switch open in run mode — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Five consecutive failures is diagnostic gold, and it shapes the bill: a condition present on every attempt gets found on the first visit ($75–150 diagnostic, usually credited if you approve the repair) rather than chased across several. The outcomes then sort by price. Free: a snow-drifted or leaf-blocked termination, or a clogged drain you clear yourself. Mid-range: pressure switch work at $150–400, where the repair guide's specific caution — check for a blocked flue before blaming the switch — is written for exactly this alert. Top of the range: a genuinely weak inducer at $300–900, with brand parts availability driving the spread. A vent-sizing problem, rarest but real on modified installs, is quoted as custom work rather than from a price book. More codes like this across brands: inducer motor codes.
DIY or call a pro
Do the outside walk first — it is the highest-yield check on Lennox's own list and needs no tools: both terminations clear of snow, ice, leaves and nests, in weather like the weather the alert appeared in. Check the condensate drain flows. If either was blocked, clear it and let the furnace retry on the next demand. Where the DIY case ends is sharper on this alert than most: five strikes means the condition is consistent, so if the terminations and drain are clean, repeatedly cycling power to force new ignition attempts is not troubleshooting — it is asking the furnace to keep testing a flue that keeps failing the test. One reset after fixing something visible is reasonable; resets in lieu of finding anything are the signal to book the visit and hand over the pressure measurements, hose inspection and inducer assessment to someone with a manometer.
The same fault on other brands
A pressure switch that will not close gets a dedicated code in nearly every control family this site documents: Carrier's code 31, Goodman's 3 flashes, and Trane's 3-flash draft pressure error all describe the same unproven draft, and the pressure switch hub collects 47 such codes across 12 families. What the Watchguard layer adds is Lennox counting the failures for you — most of those other codes fire on the first miss, while alert 271 certifies there were five in a row.
What this looks like
From the outside, this usually shows up as the inducer spins up but the burners never fire — that walkthrough covers what to check in order, and when it stops being a homeowner job.